Read operation for non-volatile storage with compensation for coupling

ABSTRACT

Shifts in the apparent charge stored on a floating gate (or other charge storing element) of a non-volatile memory cell can occur because of the coupling of an electric field based on the charge stored in adjacent floating gates (or other adjacent charge storing elements). The problem occurs most pronouncedly between sets of adjacent memory cells that have been programmed at different times. To account for this coupling, the read process for a particular memory cell will provide compensation to an adjacent memory cell in order to reduce the coupling effect that the adjacent memory cell has on the particular memory cell.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 11/384,057, “Read Operation For Non-Volatile Storage WithCompensation For Coupling,” filed on Mar. 17, 2006 by Mokhlesi,published on Sep. 6, 2007 as U.S. Publication No. US 20070206421,Attorney Docket No. SAND-01089US2, which claims the benefit of U.S.Provisional Application No. 60/778,857, filed Mar. 3, 2006. Both ofthese applications are incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following U.S. patent and U.S. patent application arecross-referenced and incorporated by reference herein in its entirety:

U.S. Pat. No. 7,436,733, issued Oct. 14, 2008, entitled “System forPerforming Read Operation On Non-volatile Storage with Compensation forCoupling,” by Nima Mokhlesi, and

U.S. patent application Ser. No. ______ [Attorney Docket No.SAND-01089US3], entitled “Read Operation For Non-Volatile Storage WithCompensation For Coupling,” by Nima Mokhlesi, filed the same day as thepresent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for non-volatile memory.

2. Description of the Related Art

Semiconductor memory has become more popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrical Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories.

Both EEPROM and flash memory utilize a floating gate that is positionedabove and insulated from a channel region in a semiconductor substrate.The floating gate is positioned between the source and drain regions. Acontrol gate is provided over and insulated from the floating gate. Thethreshold voltage of the transistor is controlled by the amount ofcharge that is retained on the floating gate. That is, the minimumamount of voltage that must be applied to the control gate before thetransistor is turned on to permit conduction between its source anddrain is controlled by the level of charge on the floating gate.

When programming an EEPROM or flash memory device, such as a NAND flashmemory device, typically a program voltage is applied to the controlgate and the bit line is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory cell is raised so that the memory cellis in a programmed state. More information about programming can befound in U.S. Pat. No. 6,859,397, titled “Source Side Self BoostingTechnique for Non-Volatile Memory;” and in U.S. Pat. No. 6,917,542,titled “Detecting Over Programmed Memory;” both patents are incorporatedherein by reference in their entirety.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory cell can beprogrammed/erased between two states (an erased state and a programmedstate). Such a flash memory device is sometimes referred to as a binaryflash memory device.

A multi-state flash memory device is implemented by identifying multipledistinct allowed/valid programmed threshold voltage ranges separated byforbidden ranges. Each distinct threshold voltage range corresponds to apredetermined value for the set of data bits encoded in the memorydevice.

Shifts in the apparent charge stored on a floating gate can occurbecause of the coupling of an electric field based on the charge storedin adjacent floating gates. This floating gate to floating gate couplingphenomena is described in U.S. Pat. No. 5,867,429, which is incorporatedherein by reference in its entirety. An adjacent floating gate to atarget floating gate may include neighboring floating gates that are onthe same bit line, neighboring floating gates on the same word line, orfloating gates that are diagonal from the target floating gate becausethey are on both a neighboring bit line and neighboring word line.

The floating gate to floating gate coupling phenomena occurs mostpronouncedly between sets of adjacent memory cells that have beenprogrammed at different times. For example, a first memory cell isprogrammed to add a level of charge to its floating gate thatcorresponds to one set of data. Subsequently, one or more adjacentmemory cells are programmed to add a level of charge to their floatinggates that correspond to a second set of data. After the one or more ofthe adjacent memory cells are programmed, the charge level read from thefirst memory cell appears to be different than programmed because of theeffect of the charge on the adjacent memory cells being coupled to thefirst memory cell. The coupling from adjacent memory cells can shift theapparent charge level being read a sufficient amount to lead to anerroneous reading of the data stored.

The effect of the floating gate to floating gate coupling is of greaterconcern for multi-state devices because in multi-state devices theallowed threshold voltage ranges and the forbidden ranges are narrowerthan in binary devices. Therefore, the floating gate to floating gatecoupling can result in memory cells being shifted from an allowedthreshold voltage range to a forbidden range.

As memory cells continue to shrink in size, the natural programming anderase distributions of threshold voltages are expected to increase dueto short channel effects, greater oxide thickness/coupling ratiovariations and more channel dopant fluctuations, reducing the availableseparation between adjacent states. This effect is much more significantfor multi-state memories than memories using only two states (binarymemories). Furthermore, the reduction of the space between word linesand of the space between bit lines will also increase the couplingbetween adjacent floating gates.

Thus, there is a need to reduce the effect of coupling between floatinggates.

SUMMARY OF THE INVENTION

To account for the coupling between floating gates, the read process fora particular memory cell will provide compensation to an adjacent memorycell in order to reduce the coupling effect that the adjacent memorycell has on the particular memory cell. Various embodiments aredisclosed.

One embodiment includes applying a read voltage to a selectednon-volatile storage element during a read process for a selectednon-volatile storage element, using a particular voltage during the readprocess for a neighbor of the selected non-volatile storage elementbased on a current condition of the neighbor, and sensing a condition ofthe selected non-volatile storage element during the read process.Another embodiment includes applying a read compare voltage to aselected word line connected to a non-volatile storage element beingread, applying a first pass voltage to a first set of unselected wordlines, applying a second pass voltage to neighbor unselected word line,and sensing a condition of the non-volatile storage element being read.

One example implementation comprises a plurality of non-volatile storageelements and one or more managing circuits in communication with theplurality of non-volatile storage elements for performing the processesdiscussed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a NAND string.

FIG. 2 is an equivalent circuit diagram of the NAND string.

FIG. 3 is a cross-sectional view of the NAND string.

FIG. 4 is a block diagram of an array of NAND flash memory cells.

FIG. 5 is a block diagram of a non-volatile memory system.

FIG. 6 is a block diagram of a non-volatile memory system.

FIG. 7 is a block diagram depicting one embodiment of the sense block.

FIG. 8 is a flow chart describing one embodiment of a process forprogramming non-volatile memory.

FIG. 9 is an example wave form applied to the control gates ofnon-volatile memory cells.

FIG. 10 is a timing diagram that explains the behavior of certainsignals during read/verify operations.

FIG. 11 depicts an example set of threshold voltage distributions.

FIG. 12 depicts an example set of threshold voltage distributions.

FIGS. 13A-C show various threshold voltage distributions and describe aprocess for programming non-volatile memory.

FIGS. 14A-G are tables depicting the order of programming non-volatilememory in various embodiments.

FIG. 15 is a flow chart describing one embodiment of a process forreading non-volatile memory.

FIG. 16 is a flow chart describing one embodiment of a process forperforming a read operation for non-volatile memory.

FIG. 17 is a flow chart describing one embodiment of a process forrecovering data.

FIG. 18 is a flow chart describing one embodiment of a process forrecovering data from multiple word lines.

FIG. 19 is a flow chart describing one embodiment of a process forreading data from a lower page.

FIG. 20 is a flow chart describing one embodiment of a process ofreading data from an upper page.

FIG. 21 is a flow chart describing one embodiment of a process forreading data.

FIG. 22 is a flow chart describing one embodiment of a process forreading data from an upper page.

FIG. 23 is a flow chart describing one embodiment of a process forreading data without using compensation.

FIG. 24 is a flow chart describing one embodiment of a process forreading data while compensating for floating gate to floating gate (ordielectric region to dielectric region) coupling.

FIG. 25 is a table depicting a process for determining data values.

FIG. 26 is a flow chart describing one embodiment of a process forreading upper page data using a correction.

FIG. 27 is a block diagram showing capacitive coupling between twoneighboring memory cells.

DETAILED DESCRIPTION

One example of a memory system suitable for implementing the presentinvention uses the NAND flash memory structure, which includes arrangingmultiple transistors in series between two select gates. The transistorsin series and the select gates are referred to as a NAND string. FIG. 1is a top view showing one NAND string. FIG. 2 is an equivalent circuitthereof. The NAND string depicted in FIGS. 1 and 2 includes fourtransistors, 100, 102, 104 and 106, in series and sandwiched between afirst select gate 120 and a second select gate 122. Select gate 120gates the NAND string connection to bit line 126. Select gate 122 gatesthe NAND string connection to source line 128. Select gate 120 iscontrolled by applying the appropriate voltages to control gate 120CG.Select gate 122 is controlled by applying the appropriate voltages tocontrol gate 122CG. Each of the transistors 100, 102, 104 and 106 has acontrol gate and a floating gate. Transistor 100 has control gate 100CGand floating gate 100FG. Transistor 102 includes control gate 102CG andfloating gate 102FG. Transistor 104 includes control gate 104CG andfloating gate 104FG. Transistor 106 includes a control gate 106CG andfloating gate 106FG. Control gate 100CG is connected to (or is) wordline WL3, control gate 102CG is connected to word line WL2, control gate104CG is connected to word line WL1, and control gate 106CG is connectedto word line WL0. In one embodiment, transistors 100, 102, 104 and 106are each memory cells. In other embodiments, the memory cells mayinclude multiple transistors or may be different than that depicted inFIGS. 1 and 2. Select gate 120 is connected to select line SGD. Selectgate 122 is connected to select line SGS.

FIG. 3 provides a cross-sectional view of the NAND string describedabove. As depicted in FIG. 3, the transistors of the NAND string areformed in p-well region 140. Each transistor includes a stacked gatestructure that consists of a control gate (100CG, 102CG, 104CG and106CG) and a floating gate (100FG, 102FG, 104FG and 106FG). The controlgates and the floating gates are typically formed by depositingpoly-silicon layers. The floating gates are formed on the surface of thep-well on top of an oxide or other dielectric film. The control gate isabove the floating gate, with an inter-polysilicon dielectric layerseparating the control gate and floating gate. The control gates of thememory cells (100, 102, 104 and 106) form the word lines. N+ dopeddiffusion regions 130, 132, 134, 136 and 138 are shared betweenneighboring cells, through which the cells are connected to one anotherin series to form a NAND string. These N+ doped regions form the sourceand drain of each of the cells. For example, N+ doped region 130 servesas the drain of transistor 122 and the source for transistor 106, N+doped region 132 serves as the drain for transistor 106 and the sourcefor transistor 104, N+ doped region 134 serves as the drain fortransistor 104 and the source for transistor 102, N+ doped region 136serves as the drain for transistor 102 and the source for transistor100, and N+ doped region 138 serves as the drain for transistor 100 andthe source for transistor 120. N+ doped region 126 connects to the bitline for the NAND string, while N+ doped region 128 connects to a commonsource line for multiple NAND strings.

Note that although FIGS. 1-3 show four memory cells in the NAND string,the use of four transistors is provided only as an example. A NANDstring used with the technology described herein can have less than fourmemory cells or more than four memory cells. For example, some NANDstrings will include 8 memory cells, 16 memory cells, 32 memory cells,64 memory cells, etc. The discussion herein is not limited to anyparticular number of memory cells in a NAND string.

Each memory cell can store data represented in analog or digital form.When storing one bit of digital data, the range of possible thresholdvoltages of the memory cell is divided into two ranges, which areassigned logical data “1” and “0.” In one example of a NAND-type flashmemory, the voltage threshold is negative after the memory cell iserased, and defined as logic “1.” The threshold voltage is positiveafter a program operation, and defined as logic “0.” When the thresholdvoltage is negative and a read is attempted by applying 0 volts to thecontrol gate, the memory cell will turn on to indicate logic one isbeing stored. When the threshold voltage is positive and a readoperation is attempted by applying 0 volts to the control gate, thememory cell will not turn on, which indicates that logic zero is stored.

A memory cell can also store multiple states, thereby storing multiplebits of digital data. In the case of storing multiple states of data,the threshold voltage window is divided into the number of states. Forexample, if four states are used, there will be four threshold voltageranges assigned to the data values “11,” “10,” “01,” and “00.” In oneexample of a NAND-type memory, the threshold voltage after an eraseoperation is negative and defined as “11.” Positive threshold voltagesare used for the states of “10,” “01,” and “00.” In someimplementations, the data values (e.g., logical states) are assigned tothe threshold ranges using a Gray code assignment so that if thethreshold voltage of a floating gate erroneously shifts to itsneighboring physical state, only one bit will be affected. The specificrelationship between the data programmed into the memory cell and thethreshold voltage ranges of the cell depends upon the data encodingscheme adopted for the memory cells. For example, U.S. Pat. No.6,222,762 and U.S. patent application Ser. No. 10/461,244, “TrackingCells For A Memory System,” filed on Jun. 13, 2003, both of which areincorporated herein by reference in their entirety, describe variousdata encoding schemes for multi-state flash memory cells.

Relevant examples of NAND-type flash memories and their operation areprovided in the following U.S. patents/patent applications, all of whichare incorporated herein by reference in their entirety: U.S. Pat. No.5,570,315; U.S. Pat. No. 5,774,397; U.S. Pat. No. 6,046,935; U.S. Pat.No. 5,386,422; U.S. Pat. No. 6,456,528; and U.S. patent application Ser.No. 09/893,277 (Publication No. US2003/0002348). Other types ofnon-volatile memory in addition to NAND flash memory can also be usedwith the present invention.

Another type of memory cell useful in flash EEPROM systems utilizes anon-conductive dielectric material in place of a conductive floatinggate to store charge in a non-volatile manner. Such a cell is describedin an article by Chan et al., “A True Single-TransistorOxide-Nitride-Oxide EEPROM Device,” IEEE Electron Device Letters, Vol.EDL-8, No. 3, March 1987, pp. 93-95. A triple layer dielectric formed ofsilicon oxide, silicon nitride and silicon oxide (“ONO”) is sandwichedbetween a conductive control gate and a surface of a semi-conductivesubstrate above the memory cell channel. The cell is programmed byinjecting electrons from the cell channel into the nitride, where theyare trapped and stored in a limited region. This stored charge thenchanges the threshold voltage of a portion of the channel of the cell ina manner that is detectable. The cell is erased by injecting hot holesinto the nitride. See also Nozaki et al., “A 1-Mb EEPROM with MONOSMemory Cell for Semiconductor Disk Application,” IEEE Journal ofSolid-State Circuits, Vol. 26, No. 4, April 1991, pp. 497-501, whichdescribes a similar cell in a split-gate configuration where a dopedpolysilicon gate extends over a portion of the memory cell channel toform a separate select transistor. The foregoing two articles areincorporated herein by reference in their entirety. The programmingtechniques mentioned in section 1.2 of “Nonvolatile Semiconductor MemoryTechnology,” edited by William D. Brown and Joe E. Brewer, IEEE Press,1998, incorporated herein by reference, are also described in thatsection to be applicable to dielectric charge-trapping devices. Thememory cells described in this paragraph can also be used with thepresent invention. Thus, the technology described herein also applies tocoupling between dielectric regions of different memory cells.

Another approach to storing two bits in each cell has been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November2000, pp. 543-545. An ONO dielectric layer extends across the channelbetween source and drain diffusions. The charge for one data bit islocalized in the dielectric layer adjacent to the drain, and the chargefor the other data bit localized in the dielectric layer adjacent to thesource. Multi-state data storage is obtained by separately readingbinary states of the spatially separated charge storage regions withinthe dielectric. The memory cells described in this paragraph can also beused with the present invention.

FIG. 4 illustrates an example of an array of NAND cells, such as thoseshown in FIGS. 1-3. Along each column, a bit line 206 is coupled to thedrain terminal 126 of the drain select gate for the NAND string 150.Along each row of NAND strings, a source line 204 may connect all thesource terminals 128 of the source select gates of the NAND strings. Anexample of a NAND architecture array and its operation as part of amemory system is found in U.S. Pat. Nos. 5,570,315; 5,774,397; and6,046,935.

The array of memory cells is divided into a large number of blocks ofmemory cells. As is common for flash EEPROM systems, the block is theunit of erase. That is, each block contains the minimum number of memorycells that are erased together. Each block is typically divided into anumber of pages. A page is a unit of programming. In one embodiment, theindividual pages may be divided into segments and the segments maycontain the fewest number of cells that are written at one time as abasic programming operation. One or more pages of data are typicallystored in one row of memory cells. A page can store one or more sectors.A sector includes user data and overhead data. Overhead data typicallyincludes an Error Correction Code (ECC) that has been calculated fromthe user data of the sector. A portion of the controller (describedbelow) calculates the ECC when data is being programmed into the array,and also checks it when data is being read from the array.Alternatively, the ECCs and/or other overhead data are stored indifferent pages, or even different blocks, than the user data to whichthey pertain.

A sector of user data is typically 512 bytes, corresponding to the sizeof a sector in magnetic disk drives. Overhead data is typically anadditional 16-20 bytes. A large number of pages form a block, anywherefrom 8 pages, for example, up to 32, 64, 128 or more pages. In someembodiments, a row of NAND strings comprises a block.

Memory cells are erased in one embodiment by raising the p-well to anerase voltage (e.g., 20 volts) for a sufficient period of time andgrounding the word lines of a selected block while the source and bitlines are floating. Due to capacitive coupling, the unselected wordlines, bit lines, select lines, and c-source are also raised to asignificant fraction of the erase voltage. A strong electric field isthus applied to the tunnel oxide layers of selected memory cells and thedata of the selected memory cells are erased as electrons of thefloating gates are emitted to the substrate side, typically byFowler-Nordheim tunneling mechanism. As electrons are transferred fromthe floating gate to the p-well region, the threshold voltage of aselected cell is lowered. Erasing can be performed on the entire memoryarray, separate blocks, or another unit of cells.

FIG. 5 illustrates a memory device 296 having read/write circuits forreading and programming a page of memory cells in parallel, according toone embodiment of the present invention. Memory device 296 may includeone or more memory die 298. Memory die 298 includes a two-dimensionalarray of memory cells 300, control circuitry 310, and read/writecircuits 365. In some embodiments, the array of memory cells can bethree dimensional. The memory array 300 is addressable by word lines viaa row decoder 330 and by bit lines via a column decoder 360. Theread/write circuits 365 include multiple sense blocks 400 and allow apage of memory cells to be read or programmed in parallel. Typically acontroller 350 is included in the same memory device 296 (e.g., aremovable storage card) as the one or more memory die 298. Commands andData are transferred between the host and controller 350 via lines 320and between the controller and the one or more memory die 298 via lines318.

The control circuitry 310 cooperates with the read/write circuits 365 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip-levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 360. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

In some implementations, some of the components of FIG. 5 can becombined. In various designs, one or more of the components of FIG. 5(alone or in combination), other than memory cell array 300, can bethought of as a managing circuit. For example, one or more managingcircuits may include any one of or a combination of control circuitry310, state machine 312, decoders 314/360, power control 316, senseblocks 400, read/write circuits 365, controller 350, etc.

FIG. 6 illustrates another arrangement of the memory device 296 shown inFIG. 5. Access to the memory array 300 by the various peripheralcircuits is implemented in a symmetric fashion, on opposite sides of thearray, so that the densities of access lines and circuitry on each sideare reduced by half. Thus, the row decoder is split into row decoders330A and 330B and the column decoder into column decoders 360A and 360B.Similarly, the read/write circuits are split into read/write circuits365A connecting to bit lines from the bottom and read/write circuits365B connecting to bit lines from the top of the array 300. In this way,the density of the read/write modules is essentially reduced by onehalf. The device of FIG. 6 can also include a controller, as describedabove for the device of FIG. 5.

FIG. 7 is a block diagram of an individual sense block 400 partitionedinto a core portion, referred to as a sense module 380, and a commonportion 390. In one embodiment, there will be a separate sense module380 for each bit line and one common portion 390 for a set of multiplesense modules 380. In one example, a sense block will include one commonportion 390 and eight sense modules 380. Each of the sense modules in agroup will communicate with the associated common portion via a data bus372. For further details refer to U.S. patent application Ser. No.11/026,536 “Non-Volatile Memory & Method with Shared Processing for anAggregate of Sense Amplifiers” filed on Dec. 29, 2004, which isincorporated herein by reference in its entirety.

Sense module 380 comprises sense circuitry 370 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. Sense module 380 also includes a bit linelatch 382 that is used to set a voltage condition on the connected bitline. For example, a predetermined state latched in bit line latch 382will result in the connected bit line being pulled to a statedesignating program inhibit (e.g., Vdd).

Common portion 390 comprises a processor 392, a set of data latches 394and an I/O Interface 396 coupled between the set of data latches 394 anddata bus 320. Processor 392 performs computations. For example, one ofits functions is to determine the data stored in the sensed memory celland store the determined data in the set of data latches. The set ofdata latches 394 is used to store data bits determined by processor 392during a read operation. It is also used to store data bits importedfrom the data bus 320 during a program operation. The imported data bitsrepresent write data meant to be programmed into the memory. I/Ointerface 396 provides an interface between data latches 394 and thedata bus 320.

During read or sensing, the operation of the system is under the controlof state machine 312 that controls the supply of different control gatevoltages to the addressed cell. As it steps through the variouspredefined control gate voltages corresponding to the various memorystates supported by the memory, the sense module 380 may trip at one ofthese voltages and an output will be provided from sense module 380 toprocessor 392 via bus 372. At that point, processor 392 determines theresultant memory state by consideration of the tripping event(s) of thesense module and the information about the applied control gate voltagefrom the state machine via input lines 393. It then computes a binaryencoding for the memory state and stores the resultant data bits intodata latches 394. In another embodiment of the core portion, bit linelatch 382 serves double duty, both as a latch for latching the output ofthe sense module 380 and also as a bit line latch as described above.

It is anticipated that some implementations will include multipleprocessors 392. In one embodiment, each processor 392 will include anoutput line (not depicted in FIG. 7) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR can determine when allbits being programmed have reached the desired level. For example, wheneach bit has reached its desired level, a logic zero for that bit willbe sent to the wired-OR line (or a data one is inverted). When all bitsoutput a data 0 (or a data one inverted), then the state machine knowsto terminate the programming process. Because each processorcommunicates with eight sense modules, the state machine needs to readthe wired-OR line eight times, or logic is added to processor 392 toaccumulate the results of the associated bit lines such that the statemachine need only read the wired-OR line one time. Similarly, bychoosing the logic levels correctly, the global state machine can detectwhen the first bit changes its state and change the algorithmsaccordingly.

During program or verify, the data to be programmed is stored in the setof data latches 394 from the data bus 320. The program operation, underthe control of the state machine, comprises a series of programmingvoltage pulses applied to the control gates of the addressed memorycells. Each programming pulse is followed by a read back (verify) todetermine if the cell has been programmed to the desired memory state.Processor 392 monitors the read back memory state relative to thedesired memory state. When the two are in agreement, the processor 222sets the bit line latch 214 so as to cause the bit line to be pulled toa state designating program inhibit. This inhibits the cell coupled tothe bit line from further programming even if programming pulses appearon its control gate. In other embodiments the processor initially loadsthe bit line latch 382 and the sense circuitry sets it to an inhibitvalue during the verify process.

Data latch stack 394 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are three data latches persense module 380. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 320, and viceversa. In the preferred embodiment, all the data latches correspondingto the read/write block of m memory cells can be linked together to forma block shift register so that a block of data can be input or output byserial transfer. In particular, the bank of r read/write modules isadapted so that each of its set of data latches will shift data in to orout of the data bus in sequence as if they are part of a shift registerfor the entire read/write block.

Additional information about the structure and/or operations of variousembodiments of non-volatile storage devices can be found in (1) UnitedStates Patent Application Pub. No. 2004/0057287, “Non-Volatile MemoryAnd Method With Reduced Source Line Bias Errors,” published on Mar. 25,2004; (2) United States Patent Application Pub No. 2004/0109357,“Non-Volatile Memory And Method with Improved Sensing,” published onJun. 10, 2004; (3) U.S. patent application Ser. No. 11/015,199 titled“Improved Memory Sensing Circuit And Method For Low Voltage Operation,”Inventor Raul-Adrian Cernea, filed on Dec. 16, 2004; (4) U.S. patentapplication Ser. No. 11/099,133, titled “Compensating for CouplingDuring Read Operations of Non-Volatile Memory,” Inventor Jian Chen,filed on Apr. 5, 2005; and (5) U.S. patent application Ser. No.11/321,953, titled “Reference Sense Amplifier For Non-Volatile Memory,Inventors Siu Lung Chan and Raul-Adrian Cemea, filed on Dec. 28, 2005.All five of the immediately above-listed patent documents areincorporated herein by reference in their entirety.

FIG. 8 is a flow chart describing one embodiment of a method forprogramming non-volatile memory. In one implementation, memory cells areerased (in blocks or other units) prior to programming. In step 400 ofFIG. 8, a “data load” command is issued by the controller and inputreceived by control circuitry 310. In step 402, address data designatingthe page address is input to decoder 314 from the controller or host. Instep 404, a page of program data for the addressed page is input to adata buffer for programming. That data is latched in the appropriate setof latches. In step 406, a “program” command is issued by the controllerto state machine 312.

Triggered by the “program” command, the data latched in step 404 will beprogrammed into the selected memory cells controlled by state machine312 using the stepped pulses of FIG. 9 applied to the appropriate wordline. In step 408, the program voltage Vpgm is initialized to thestarting pulse (e.g., 12V or other value) and a program counter PCmaintained by state machine 312 is initialized at 0. In step 410, thefirst Vpgm pulse is applied to the selected word line. If logic “0” isstored in a particular data latch indicating that the correspondingmemory cell should be programmed, then the corresponding bit line isgrounded. On the other hand, if logic “1” is stored in the particularlatch indicating that the corresponding memory cell should remain in itscurrent data state, then the corresponding bit line is connected to Vddto inhibit programming.

In step 412, the states of the selected memory cells are verified. If itis detected that the target threshold voltage of a selected cell hasreached the appropriate level, then the data stored in the correspondingdata latch is changed to a logic “1.” If it is detected that thethreshold voltage has not reached the appropriate level, the data storedin the corresponding data latch is not changed. In this manner, a bitline having a logic “1” stored in its corresponding data latch does notneed to be programmed. When all of the data latches are storing logic“1,” the state machine (via the wired-OR type mechanism described above)knows that all selected cells have been programmed. In step 414, it ischecked whether all of the data latches are storing logic “1.” If so,the programming process is complete and successful because all selectedmemory cells were programmed and verified. A status of “PASS” isreported in step 416. In one embodiment, the verification of step 412includes providing a different one or more voltages to memory cellsadjacent to the memory cells being programmed than that which isprovided to the other unselected memory cells. For example, if memorycells on word line WLn are being programmed, then the voltage applied tomemory cells on word lines WLn+1 will be different than the voltageapplied to other unselected word lines. This compensation will bediscussed in more detail below with respect to FIG. 10.

If, in step 414, it is determined that not all of the data latches arestoring logic “1,” then the programming process continues. In step 418,the program counter PC is checked against a program limit value PCMAX.One example of a program limit value is 20; however, other numbers canalso be used. If the program counter PC is not less than 20, then theprogram process has failed and a status of “FAIL” is reported in step420. If the program counter PC is less than 20, then the Vpgm level isincreased by the step size and the program counter PC is incremented instep 422. After step 422, the process loops back to step 410 to applythe next Vpgm pulse.

FIG. 9 shows a series of program pulses that are applied to the wordline selected for programming. In between program pulses are a set ofverify pulses (not depicted). In some embodiments, there can be a verifypulse for each state that data is being programmed into. In otherembodiments, there can be more or less verify pulses.

In one embodiment, data is programmed to memory cells along a commonword line. Thus, prior to applying the program pulses of FIG. 9, one ofthe word lines is selected for programming. This word line will bereferred to as the selected word line. The remaining word lines of ablock are referred to as the unselected word lines. The selected wordline may have one or two neighboring word lines. If the selected wordline has two neighboring word lines, then the neighboring word line onthe drain side is referred to as the drain side neighboring word lineand the neighboring word line on the source side is referred to as thesource side neighboring word line. For example, if WL2 of FIG. 2 is theselected word line, then WL1 is the source side neighboring word lineand WL3 is the drain side neighboring word line.

Each block of memory cells includes a set of bit lines forming columnsand a set of word lines forming rows. In one embodiment, the bit linesare divided into odd bit lines and even bit lines. Memory cells along acommon word line and connected to the odd bit lines are programmed atone time, while memory cells along a common word line and connected toeven bit lines are programmed at another time (“odd/even programming”).In another embodiment, memory cells are programmed along a word line forall bit lines in the block (“all bit line programming”). In otherembodiments, the bit lines or block can be broken up into othergroupings (e.g., left and right, more than two groupings, etc.).

FIG. 10 is a timing diagram depicting the behavior of various signalsduring one iteration of a read or verify process. For example, if thememory cells are binary memory cells, the process of FIG. 10 may beperformed once for each memory cell during an iteration of step 412. Ifthe memory cells are multi-state memory cells with four states (e.g., E,A, B, and C), the process of FIG. 10 may be performed three times foreach memory cell during an iteration of step 412.

In general, during the read and verify operations, the selected wordline is connected to a voltage, a level of which is specified for eachread and verify operation in order to determine whether a thresholdvoltage of the concerned memory cell has reached such level. Afterapplying the word line voltage, the conduction current of the memorycell is measured to determine whether the memory cell turned on inresponse to the voltage applied to the word line. If the conductioncurrent is measured to be greater than a certain value, then it isassumed that the memory cell turned on and the voltage applied to theword line is greater than the threshold voltage of the memory cell. Ifthe conduction current is not measured to be greater than the certainvalue, then it is assumed that the memory cell did not turn on and thevoltage applied to the word line is not greater than the thresholdvoltage of the memory cell.

There are many ways to measure the conduction current of a memory cellduring a read or verify operation. In one example, the conductioncurrent of a memory cell is measured by the rate it discharges adedicated capacitor in the sense amplifier. In one embodiment, a memoryarray that uses all bit line programming can measure the conductioncurrent of a memory cell by the rate it discharges a dedicated capacitorin the sense amplifier. In another example, the conduction current ofthe selected memory cell allows (or fails to allow) the NAND string thatincluded the memory cell to discharge the bit line. The charge on thebit line is measured after a period of time to see whether it has beendischarged or not. In one embodiment, a memory array that uses odd/evenprogramming can measure the conduction current of a memory cell bydetermining whether the bit line has discharged. FIG. 10 explains bothexamples.

FIG. 10 shows signals SGD, WL_unsel. WLn+1, WLn, SGS, Selected BL,BLCLAMP, and Source starting at Vss (approximately 0 volts). SGDrepresents the gate of the drain side select gate. SGS is the gate ofthe source side select gate. WLn is the word line selected forreading/verification. WLn+1 is the unselected word line that is thedrain side neighboring word line to WLn. WL_unsel represents theunselected word lines other than the drain side neighboring word line.Selected BL is the bit line selected for reading/verification. Source isthe source line for the memory cells (see FIG. 4). BLCLAMP is an analogsignal that sets the value of the bit line when charged from the senseamplifier. Note that there are two versions of SGS, Selected BL andBLCLAMP depicted. One set of these signals SGS (B), Selected BL (B) andBLCLAMP (B) depict a read/verify operation for an array of memory cellsthat measure the conduction current of a memory cell by determiningwhether the bit line has discharged. Another set of these signals SGS(C), Selected BL (C) and BLCLAMP (C) depict a read/verify operation foran array of memory cells that measure the conduction current of a memorycell by the rate it discharges a dedicated capacitor in the senseamplifier.

First, the behavior of the sensing circuits and the array of memorycells that are involved in measuring the conduction current of a memorycell by determining whether the bit line has discharged will bediscussed with respect to SGS (B), Selected BL (B), and BLCLAMP (B). Attime t1 of FIG. 10, SGD is raised to Vdd (e.g., approximately 3.5volts), the unselected word lines (WL_unsel) are raised to Vread (e.g.,approximately 5.5 volts), the drain side neighboring word line (WLn+1)is raised to VreadX, the selected word line WLn is raised to Vcgr (e.g.,Vra, Vrb, or Vrc of FIG. 11) for a read operation or a verify level(e.g., Vva, Vvb, or Vvc of FIG. 11) for a verify operation, and BLCLAMP(B) is raised to a pre-charging voltage to pre-charge the selected bitline Selected BL(B) (e.g., to approximately 0.7 volts). The voltagesVread and VreadX act as pass voltages because they cause the unselectedmemory cells to turn on and act as pass gates. At time t2, BLCLAMP (B)is lowered to Vss so the NAND string can control the bit line. Also attime t2, the source side select gate is turned on by raising SGS (B) toVdd. This provides a path to dissipate the charge on the bit line. Ifthe threshold voltage of the memory cell selected for reading is greaterthan Vcgr or the verify level applied to the selected word line WLn,then the selected memory cell will not turn on and the bit line will notdischarge, as depicted by signal line 450. If the threshold voltage inthe memory cell selected for reading is below Vcgr or below the verifylevel applied to the selected word line WLn, then the memory cellselected for reading will turn on (conduct) and the bit line voltagewill dissipate, as depicted by curve 452. At some point after time t2and prior to time t3 (as determined by the particular implementation),the sense amplifier will determine whether the bit line has dissipated asufficient amount. In between t2 and t3, BLCLAMP (B) is raised to letthe sense amplifier measure the evaluated BL voltage and then lowered,as depicted in FIG. 10. At time t3, the depicted signals will be loweredto Vss (or another value for standby or recovery). Note that in otherembodiments, the timing of some of the signals can be changed (e.g.shift the signal applied to the neighbor).

Next, the behavior of the sensing circuits and the array of memory cellsthat measure the conduction current of a memory cell by the rate itdischarges a dedicated capacitor in the sense amplifier will bediscussed with respect to SGS (C), Selected BL (C) and BLCLAMP (C). Attime t1 of FIG. 10, SGD is raised to Vdd (e.g., approximately 3.5volts), the unselected word lines (WL_unsel) are raised to Vread (e.g.,approximately 5.5 volts), the drain side neighboring word line (WLn+1)is raised to VreadX, the selected word line WLn is raised to Vcgr (e.g.,Vra, Vrb, or Vrc of FIG. 11) for a read operation or a verify level(e.g., Vva, Vvb, or Vvc of FIG. 11) for a verify operation, and BLCLAMP(C) is raised. In this case, the sense amplifier holds the bit linevoltage constant regardless of what the NAND sting is doing, so thesense amplifier measures the current flowing with the bit line “clamped”to that voltage. Therefore, BLCLAMP (C) rises at t1 and does not changefrom t1 to t3. At some point after time t1 and prior to time t3 (asdetermined by the particular implementation), the sense amplifier willdetermine whether the capacitor in the sense amplifier has dissipated asufficient amount. At time t3, the depicted signals will be lowered toVss (or another value for standby or recovery). Note that in otherembodiments, the timing of some of the signals can be changed.

As discussed above, shifts in the apparent threshold voltage of afloating gate (or other charge storing element) of a non-volatile memorycell as measured from the control gate can occur because of the couplingof an electric field based on the charge stored in adjacent floatinggates (or other adjacent charge storing elements). The problem occursmost pronouncedly between sets of adjacent memory cells that have beenprogrammed at different times. To account for this coupling, the readprocess for a particular memory cell will provide compensation to anadjacent memory cell in order to reduce the coupling effect that theadjacent memory cell has on the particular memory cell. One embodimentalso includes setting up, during the verification process, the requiredconditions for the later application of compensation to the adjacentmemory cell. In such an embodiment the overdrive/bypass voltage,otherwise known as VREAD, applied to WLn+1 is reduced from a typicalvalue of, for example, 6V down to, for example, 3V. The compensationwill consist of application of higher voltage, as compared to thatvoltage that was used during the verify phase of program/verifyoperations, to WLn+1 during the read operation performed on WLn. Inother words the compensation consists of a change/delta:ΔVREAD={[VREAD(WLn+1 during read of WLn)]−[VREAD(WLn+1 during verify ofWLn)]}. The advantage of using a lower VREAD value during verify is thatit allows the application of nominal values of VREAD later during readoperations, while maintaining the required ΔVREAD. Had it not been forthe use of a smaller than nominal value of VREAD during verify, thenecessary value of VREAD during read that would allow the application ofsufficient ΔVREAD would have been, for example, 6+3=9V which would havebeen too high a voltage as such high VREAD voltage lead to read disturbconditions. One example of such setting up for later compensation isdepicted in FIG. 10 as the application of VreadX to the drain sideneighboring word line while the other unselected word lines receiveVread. In many prior art devices, all of the unselected word lines wouldreceive Vread. In the embodiment of FIG. 10, all of the unselected wordlines, except for the drain side neighbor, receive Vread; while thedrain side neighbor receives VreadX.

For the verify process where memory cells are programmed from the sourceside to the drain side, it is guaranteed (in one embodiment) that whenwriting to word line WLn, all memory cells on word lines WLn+1 are inthe erased state (e.g., state E) (Note: This is true for full sequenceand not for LM mode. Please see above explanation). Word line WLn+1 willreceive a voltage level VreadX, where VreadX=Vread4 (discussed below).In one embodiment, Vread4 is equal to 3.7 v. In another embodiment,VreadX=Vread. In other embodiment, other values can also be used. Indifferent implementations, different values of Vread4 or VreadX can bedetermined based on device characterization, experimentation and/orsimulation.

At the end of a successful program process, the threshold voltages ofthe memory cells should be within one or more distributions of thresholdvoltages for programmed memory cells or within a distribution ofthreshold voltages for erased memory cells, as appropriate. FIG. 11illustrates example threshold voltage distributions for the memory cellarray when each memory cell stores two bits of data. FIG. 11 shows afirst threshold voltage distribution E for erased memory cells. Threethreshold voltage distributions, A, B and C for programmed memory cells,are also depicted. In one embodiment, the threshold voltages in the Edistribution are negative and the threshold voltages in the A, B and Cdistributions are positive.

Each distinct threshold voltage range of FIG. 11 corresponds topredetermined values for the set of data bits. The specific relationshipbetween the data programmed into the memory cell and the thresholdvoltage levels of the cell depends upon the data encoding scheme adoptedfor the cells. For example, U.S. Pat. No. 6,222,762 and U.S. patentapplication Ser. No. 10/461,244, “Tracking Cells For A Memory System,”filed on Jun. 13, 2003, both of which are incorporated herein byreference in their entirety, describe various data encoding schemes formulti-state flash memory cells. In one embodiment, data values areassigned to the threshold voltage ranges using a Gray code assignment sothat if the threshold voltage of a floating gate erroneously shifts toits neighboring physical state, only one bit will be affected. Oneexample assigns “11” to threshold voltage range E (state E), “10” tothreshold voltage range A (state A), “00” to threshold voltage range B(state B) and “01” to threshold voltage range C (state C). However, inother embodiments, Gray code is not used. Although FIG. 11 shows fourstates, the present invention can also be used with other multi-statestructures including those that include more or less than four states.

FIG. 11 also shows three read reference voltages, Vra, Vrb and Vrc, forreading data from memory cells. By testing whether the threshold voltageof a given memory cell is above or below Vra, Vrb and Vrc, the systemcan determine what state the memory cell is in.

FIG. 11 also shows three verify reference voltages, Vva, Vvb and Vvc.When programming memory cells to state A, the system will test whetherthose memory cells have a threshold voltage greater than or equal toVva. When programming memory cells to state B, the system will testwhether the memory cells have threshold voltages greater than or equalto Vvb. When programming memory cells to state C, the system willdetermine whether memory cells have their threshold voltage greater thanor equal to Vvc.

In one embodiment, known as full sequence programming, memory cells canbe programmed from the erase state E directly to any of the programmedstates A, B or C. For example, a population of memory cells to beprogrammed may first be erased so that all memory cells in thepopulation are in erased state E. The process depicted in FIG. 18, usingthe control gate voltage sequence depicted in FIG. 9, will then be usedto program memory cells directly into states A, B or C. While somememory cells are being programmed from state E to state A, other memorycells are being programmed from state E to state B and/or from state Eto state C. When programming from state E to state C on WLn, the amountof parasitic coupling to the adjacent floating gate under WLn−1 is amaximized since the change in amount of charge on the floating gateunder WLn is largest as compared to the change in voltage whenprogramming from state E to state A or state E to state B. Whenprogramming from state E to state B the amount of coupling to theadjacent floating gate is reduced but still significant. Whenprogramming from state E to state A the amount of coupling is reducedeven further. Consequently the amount of correction required tosubsequently read each state of WLn−1 will vary depending on the stateof the adjacent cell on WLn.

FIG. 12 illustrates an example of a two-pass technique of programming amulti-state memory cell that stores data for two different pages: alower page and an upper page. Four states are depicted: state E (11),state A (10), state B (00) and state C (01). For state E, both pagesstore a “1.” For state A, the lower page stores a “0” and the upper pagestores a “1.” For state B, both pages store “0.” For state C, the lowerpage stores “1” and the upper page stores “0.” Note that althoughspecific bit patterns have been assigned to each of the states,different bit patterns may also be assigned.

In a first programming pass, the cell's threshold voltage level is setaccording to the bit to be programmed into the lower logical page. Ifthat bit is a logic “1,” the threshold voltage is not changed since itis in the appropriate state as a result of having been earlier erased.However, if the bit to be programmed is a logic “0,” the threshold levelof the cell is increased to be state A, as shown by arrow 530. Thatconcludes the first programming pass.

In a second programming pass, the cell's threshold voltage level is setaccording to the bit being programmed into the upper logical page. Ifthe upper logical page bit is to store a logic “1,” then no programmingoccurs since the cell is in one of the states E or A, depending upon theprogramming of the lower page bit, both of which carry an upper page bitof “1.” If the upper page bit is to be a logic “0,” then the thresholdvoltage is shifted. If the first pass resulted in the cell remaining inthe erased state E, then in the second phase the cell is programmed sothat the threshold voltage is increased to be within state C, asdepicted by arrow 534. If the cell had been programmed into state A as aresult of the first programming pass, then the memory cell is furtherprogrammed in the second pass so that the threshold voltage is increasedto be within state B, as depicted by arrow 532. The result of the secondpass is to program the cell into the state designated to store a logic“0” for the upper page without changing the data for the lower page. Inboth FIG. 11 and FIG. 12 the amount of coupling to the floating gate onthe adjacent word line depends on the final state.

In one embodiment, a system can be set up to perform full sequencewriting if enough data is written to fill up an entire page. If notenough data is written for a full page, then the programming process canprogram the lower page programming with the data received. Whensubsequent data is received, the system will then program the upperpage. In yet another embodiment, the system can start writing in themode that programs the lower page and convert to full sequenceprogramming mode if enough data is subsequently received to fill up anentire (or most of a) word line's memory cells. More details of such anembodiment are disclosed in U.S. patent application titled “PipelinedProgramming of Non-Volatile Memories Using Early Data,” Ser. No.11/013,125, filed on Dec. 14, 2004, inventors Sergy AnatolievichGorobets and Yan Li, incorporated herein by reference in its entirety.

FIGS. 13A-C disclose another process for programming non-volatile memorythat reduces the effect of floating gate to floating gate coupling by,for any particular memory cell, writing to that particular memory cellwith respect to a particular page subsequent to writing to adjacentmemory cells for previous pages. In one example of an implementation ofthe process taught by FIGS. 13A-C, the non-volatile memory cells storetwo bits of data per memory cell, using four data states. For example,assume that state E is the erased state and states A, B and C are theprogrammed states. State E stores data 11. State A stores data 01. StateB stores data 10. State C stores data 00. This is an example of non-Graycoding because both bits change between adjacent states A & B. Otherencodings of data to physical data states can also be used. Each memorycell stores two pages of data. For reference purposes these pages ofdata will be called upper page and lower page; however, they can begiven other labels. With reference to state A for the process of FIGS.13A-C, the upper page stores bit 0 and the lower page stores bit 1. Withreference to state B, the upper page stores bit 1 and the lower pagestores bit 0. With reference to state C, both pages store bit data 0.

The programming process of FIGS. 13A-C is a two-step process. In thefirst step, the lower page is programmed. If the lower page is to remaindata 1, then the memory cell state remains at state E. If the data is tobe programmed to 0, then the threshold of voltage of the memory cell israised such that the memory cell is programmed to state B′. FIG. 13Atherefore shows the programming of memory cells from state E to stateB′. State B′ depicted in FIG. 13A is an interim state B; therefore, theverify point is depicted as Vvb′, which is lower than Vvb.

In one embodiment, after a memory cell is programmed from state E tostate B′, its neighbor memory cell (WLn+1) in the NAND string will thenbe programmed with respect to its lower page. For example, looking backat FIG. 2, after the lower page for memory cell 106 is programmed, thelower page for memory cell 104 would be programmed. After programmingmemory cell 104, the floating gate to floating gate coupling effect willraise the apparent threshold voltage of memory cell 106 if memory cell104 had a threshold voltage raised from state E to state B′. This willhave the effect of widening the threshold voltage distribution for stateB′ to that depicted as threshold voltage distribution 550 of FIG. 13B.This apparent widening of the threshold voltage distribution will beremedied when programming the upper page.

FIG. 13C depicts the process of programming the upper page. If thememory cell is in erased state E and the upper page is to remain at 1,then the memory cell will remain in state E. If the memory cell is instate E and its upper page data is to be programmed to 0, then thethreshold voltage of the memory cell will be raised so that the memorycell is in state A. If the memory cell was in intermediate thresholdvoltage distribution 550 and the upper page data is to remain at 1, thenthe memory cell will be programmed to final state B. If the memory cellis in intermediate threshold voltage distribution 550 and the upper pagedata is to become data 0, then the threshold voltage of the memory cellwill be raised so that the memory cell is in state C. The processdepicted by FIGS. 13A-C reduces the effect of floating gate to floatinggate coupling because only the upper page programming of neighbor memorycells will have an effect on the apparent threshold voltage of a givenmemory cell. An example of an alternate state coding is to move fromdistribution 550 to state C when the upper page data is a 1, and to moveto state B when the upper page data is a 0.

Although FIGS. 13A-C provide an example with respect to four data statesand two pages of data, the concepts taught by FIGS. 13A-C can be appliedto other implementations with more or less than four states anddifferent than two pages.

FIGS. 14A-F depict various tables that describe the order of programmingaccording to various embodiments for the methods described by FIGS. 11,12 and 13A-C.

FIG. 14A is a table which describes the order for programming memorycells along a bit line for all bit line programming. In this embodiment,the block with four word lines includes four pages (page 0-3). Page 0 iswritten first, followed by page 1, followed by page 2 and then followedby page 3. The data in page 0 includes the data stored by all the memorycells connected to word line WL0. The data in page 1 includes the datastored by the memory cells connected to word line WL1. The data in page2 includes the data stored by memory cells connected to WL2. The data inpage 3 includes the data stored by memory cells connected to word lineWL3. The embodiment of FIG. 14A assumes full sequence programming, asdescribed above with respect to FIG. 11.

FIG. 14B depicts the order of programming during odd/even programmingwhen using the full sequence programming method described above withrespect to FIG. 11. In this embodiment, a block with four word linesincludes eight pages of data. The memory cells on even bit linesconnected to word line WL0 store data for page 0. Memory cells on oddbit lines connected to word line WL0 store data for page 1. Memory cellson even bit lines connected to word line WL1 store data for page 2.Memory cells on odd bit lines connected to word line WL1 store data forpage 3. Memory cells on even bit lines connected to word line WL2 storedata for page 4. Memory cells on odd bit lines connected to word lineWL2 store data for page 5. Memory cells on even bit lines connected toword line WL3 store data for page 6. Memory cells on odd bit linesconnected to word line WL3 store data for page 7. Data is programmed innumerical order according to page number, from page 0 to page 7.

The table of FIG. 14C describes the order for programming according tothe two phase programming process of FIG. 12 for a memory array thatperforms all bit line programming. A block with four word lines isdepicted to include eight pages. For memory cells connected to word lineWL0, the lower page of data forms page 0 and the upper page data formspage 1. For memory cells connected to word line WL1, the lower page ofdata forms page 2 and the upper page data forms page 3. For memory cellsconnected to word line WL2, the lower page of data forms page 4 and theupper page data forms page 5. For memory cells connected to word lineWL3, the lower page of data forms page 6 and the upper page data formspage 7. Data is programmed in numerical order according to page number,from page 0 to page 7.

FIG. 14D provides a table describing the order of programming thetwo-phase programming process of FIG. 12 for a memory architecture thatperforms odd/even programming. A block with four word lines includes 16pages, where the pages are programmed in numerical order according topage number, from page 0 to page 15. For memory cells on even bit linesconnected to word line WL0, the lower page of data forms page 0 and theupper page data forms page 2. For memory cells on odd bit linesconnected to word line WL0, the lower page of data forms page 1 and theupper page of data forms page 3. For memory cells on even bit linesconnected to word line WL1, the lower page forms page 4 and the upperpage forms page 6. For memory cells on odd bit lines connected to wordline WL1, the lower page forms page 5 and the upper page forms page 7.For memory cells on even bit lines connected to word line WL2, the lowerpage forms page 8 and the upper page forms page 10. For memory cells onodd bit lines connected to word line WL2, the lower page forms page 9and the upper page forms page 11. For memory cells on even bit linesconnected to word line WL3, the lower page forms page 12 and the upperpage forms page 14. For memory cells on odd bit lines connected to wordline WL3, the lower page forms page 13 and the upper page forms page 15.Alternately, as in FIG. 14E, both lower and upper pages under each wordline of the even bit lines are programmed before programming both pagesof the odd bit lines for this same word line.

FIGS. 14F and 14G describe the order for programming memory cellsutilizing the programming method of FIGS. 13A-C. FIG. 14F pertains tothe architecture that performs all bit line programming. For memorycells connected to word line WL0, the lower page forms page 0 and theupper page forms page 2. For memory cells connected to word line WL1,the lower page forms page 1 and the upper page forms page 4. For memorycells connected to word line WL2, the lower page forms page 3 and theupper page forms page 6. For memory cells connected to word line WL3,the lower page forms page 5 and the upper page forms page 7. Memorycells are programmed in numerical order according to page number, frompage 0 to page 7.

The table of FIG. 14G pertains to the architecture that performsodd/even programming. For memory cells on even bit lines connected toword line WL0, the lower page forms page 0 and the upper page forms page4. For memory cells on odd bit lines connected to word line WL0, thelower page forms page 1 and the upper page forms page 5. For memorycells on even bit lines connected to word line WL1, the lower page formspage 2 and the upper page forms page 8. For the memory cells on odd bitlines connected to word line WL1, the lower page forms page 3 and theupper page forms page 9. For the memory cells on even bit linesconnected to word line WL2, the lower page forms page 6 and the upperpage forms page 12. For the memory cells on odd bit lines connected toword line WL2, the lower page forms page 7 and the upper page forms page13. For the memory cells on even bit lines connected to word line WL3,the lower page forms page 10 and the upper page forms page 14. For thememory cells on odd bit lines connected to word line WL3, the lower pageforms page 11 and the upper page forms page 15. Memory cells areprogrammed in numerical order according to page number, from page 0 topage 15. Finally, each of the architectures having both even and odd bitlines can be implemented with all the even bit lines located physicallytogether in, for example, the left side of the chip, and all of the oddbit lines located together in, for example, the right side of the chip.

Note that in the embodiments of FIGS. 14A-G, memory cells are programmedalong a NAND string from source side to the drain side. Also, the tablesdepict only an embodiment with four word lines. The various methodsdepicted within the tables can be applied to systems with more or lessthan four word lines. Examples of an architecture using odd/evenprogramming can be found in U.S. Pat. Nos. 6,522,580 and 6,643,188; bothof which are incorporated herein by reference in their entirety. Moreinformation about an architecture that uses all bit line programming canbe found in the following U.S. patent documents incorporated byreference in their entirety: United States Patent ApplicationPublication US 2004/0057283; United States Patent ApplicationPublication US 2004/0060031; United States Patent ApplicationPublication US 2004/0057285; United States Patent ApplicationPublication US 2004/0057287; United States Patent ApplicationPublication US 2004/0057318; U.S. Pat. No. 6,771,536; U.S. Pat. No.6,781,877.

Generally, architectures that program all bit lines together will readdata from all bit lines together. Similarly, architectures that programodd and even bit lines separately will generally read odd and even bitlines separately. However, such limitations are not required. Thetechnology described herein for reading data can be used with all bitline programming or odd/even bit line programming. The technologydescribed herein for reading data can also be used for any of theprogramming schemes of FIGS. 17-19, as well as other programmingschemes.

FIG. 15 is a flow chart describing one embodiment for reading data fromnon-volatile memory cells. FIG. 15 provides the read process at thesystem level. In step 598, a request to read data is received. In step600, a read operation is performed for a particular page in response tothe request to read data (step 598). In one embodiment, when data for apage is programmed, the system will also create extra bits used forError Correction Codes (ECCs) and write those ECC bits along with thepage of data. ECC technologies are well known in the art. The ECCprocess used can include any suitable ECC process known in the art. Whenreading data from a page, the ECC bits will be used to determine whetherthere are any errors in the data (step 602). The ECC process can beperformed by the controller, the state machine or elsewhere in thesystem. If there are no errors in the data, the data is reported to theuser at step 604. For example, data will be communicated to a controlleror host via data I/O lines 320. If an error is found at step 602, it isdetermined whether the error is correctable (step 606). The error may bedue to the floating gate to floating gate coupling effect or otherreasons. Various ECC methods have the ability to correct a predeterminednumber of errors in a set of data. If the ECC process can correct thedata, then the ECC process is used to correct that data in step 608 andthe data, as corrected, is reported to the user in step 610. If the datais not correctable by the ECC process, a data recovery process isperformed in step 620. In some embodiments, an ECC process will beperformed after step 620. More details about the data recovery processare described below. After the data is recovered, that data is reportedat step 622. Note that the process of FIG. 15 can be used with dataprogrammed using all bit line programming or odd/even bit lineprogramming.

FIG. 16 is a flow chart describing one embodiment of a process forperforming a read operation for a page (see step 600 of FIG. 15). Theprocess of FIG. 16 can be performed for a page that encompasses all bitlines of a block, only odd bit lines of a block, only even bit lines ofa block, or other subsets of bit lines of a block. In step 640, readreference voltage Vra is applied to the appropriate word line associatedwith the page. In step 642, the bit lines associated with the page aresensed to determine whether the addressed memory cells turn on or do notturn on based on the application of Vra to their control gates. Bitlines that conduct indicate that the memory cells were turned on;therefore, the threshold voltages of those memory cells are below Vra(e.g., in state E). In step 644 the result of the sensing for the bitlines is stored in the appropriate latches for those bit lines. In step646, read reference voltage Vrb is applied to the word lines associatedwith the page being read. In step 648, the bit lines are sensed asdescribed above. In step 650, the results are stored in the appropriatelatches for the bit lines. In step 652, read reference voltage Vrc isapplied to the word lines associated with the page. In step 654, the bitlines are sensed to determine which memory cells turn on, as describedabove. In step 656, the results from the sensing step are stored in theappropriate latches for the bit lines. In step 658, the data values foreach bit line are determined. For example, if a memory cell conducts atVra, then the memory cell is in state E. If a memory cell conducts atVrb and Vrc but not at Vra, then the memory cell is in state A. If thememory cell conducts at Vrc but not at Vra and Vrb, then the memory cellis in state B. If the memory cell does not conduct at Vra, Vrb or Vrc,then the memory cell is in state C. In one embodiment, the data valuesare determined by processor 392. In step 660, processor 392 will storethe determined data values in the appropriate latches for each bit line.In other embodiments, sensing the various levels (Vra, Vrb, and Vrc) mayoccur in different orders.

Steps 640-644 include performing the operation depicted in FIG. 10, withVcgr=Vra and VreadX=Vread. Steps 646-650 include performing theoperation depicted in FIG. 10, with Vcgr=Vrb and VreadX=Vread. Steps652-656 include performing the operation depicted in FIG. 10, withVcgr=Vrc and VreadX=Vread. Thus, one embodiment of the process of FIG.16 does not include performing any compensation for floating gate tofloating gate coupling. In another embodiment, steps 640, 646, and 652are performed with VreadX=Vread4 (or another value) applied to drainside neighbor WL (i.e. WLn+1).

FIG. 17 includes a flow chart describing one embodiment of a process forrecovering data (step 620). Data may include an error due to thefloating gate to floating gate coupling effect (or another cause). Theprocess of FIG. 17 attempts to read the data while compensating for thefloating gate to floating gate coupling effect (or another cause oferror). The compensation includes looking at the neighboring word lineand determining how the programming of the neighboring word line hascreated a floating gate to floating gate coupling effect. For example,when reading data on word line WLn (e.g., WL2 of FIG. 2), the processwill also read the data of word line WLn+1 (e.g., WL3 of FIG. 2). If thedata on word line WLn+1 has caused an apparent change in the data onWLn, then the read process will compensate for that unintentionalchange.

The process depicted in FIG. 17 applies to the full sequence programmingdescribed above with respect to FIG. 11 in which two bits of one logicalpage are stored in each cell and will be read and reported out together.If the memory cell on the neighboring word line is in state E, therewill be no floating gate to floating gate coupling effect. If the memorycell on the neighboring word line is in state A, there will be a smallcoupling effect. If the memory cell on the neighboring word line is instate B, there will be a medium floating gate to floating gate couplingeffect. If the memory cell on the neighboring word line is in state C,there will be a larger floating gate to floating gate coupling effect.The exact coupling effect due to the neighboring word line varies byarray implementation and can be determined by characterizing the device.

Step 670 in FIG. 17 includes performing a read operation for theneighboring word line WLn+1. This includes performing the process ofFIG. 16 for the neighboring word line. For example, if a page in wordline WL1 is being read, then step 670 includes performing the process ofFIG. 16 on word line WL2. The results of step 670 are stored in theappropriate latches in step 672. In some embodiments, the read operationperformed for WLn+1 results in determining the actual data stored onWLn+1. In other embodiments, the read operation performed for WLn+1results in a determination of charge levels on WLn+1, which may or maynot accurately reflect the data stored on WLn+1.

When the objective is to read data on WLn, it may not necessary to havean ECC correct read of WLn+1, as bits that are read erroneously are mostprobably bits at tails of distributions, and to have mistaken them asbelonging to another data state does not cause a big error indetermining the required amount of compensation for reading thecorresponding cell(s) on WLn. For example, a slightly over programmedcell on WLn+1 which was meant to be programmed to State B, havingsubsequently experienced the capacitive coupling effect duringprogramming of WLn+2, may now be misread as being in state C when WLn+1is read without coupling compensation (step 670 of FIG. 17) as part ofthe reading process of WLn. This misreading is not an issue for thefollowing reasons: 1) the objective is not to read data on WLn+1, 2) thecorrection applied for read of corresponding cell on WLn based onapparent state of cell on WLn+1 being C-state is actually a bettercorrection than one that would have been based on the correct read ofcell on WLn+1, namely state B. This is because all the causes for thecell on WLn+1 being misread as being in state C, whether they beover-programming in the first place, or subsequent coupling from WLn+2cell, are presently at work to induce stronger coupling effect inducedby WLn+1 cell and experienced by WLn cell. Faced with this strongercoupling experienced by cell on WLn it may actually be better to applythe correction corresponding to WLn+1 cell being in state C, rather thanstate B. An alternative embodiment includes margining of read voltagesduring the read of step 670 of FIG. 17. This margining of the read ofstep 670 would be done with the intent of making coupling correctionsfor the read of step 670. But such an embodiment may be inferior to notmaking the coupling correction during read of step 670, as explainedabove.

In step 674, a read process is performed for the word line of interestWLn. This includes performing the process of FIG. 16 with VreadX=Vread1.In one embodiment, Vread1=Vread. Thus, all of the unselected word lines(see WL_unsel and WLn+1 of FIG. 10) are receiving Vread. This providesthe maximum compensation as the compensation is determined by thedifference between Vread value used on WLn+1 now during read operationsand the Vread value used earlier during the verify phase ofprogram/verify. The compensation value, compC, can be defined asfollows: compC=Vread1−Vreadp=5.5−3=2.5 v, where Vreadp is the Vreadvalue used during program/verify. The results of step 674 are stored inthe appropriate latches for bit lines with memory cells where neighborcell WLn+1 was determined (in step 670) to be in state C. Therefore, themaximum compensation, CompC, is engaged for cells whose drain sideneighbors had experienced the highest change in threshold voltage bybeing programmed from state E to state C. Note that these drain sideneighbors were in State E during program/verify of WLn, but now are inState C. What has to be compensated for under all circumstances is thechange in state of the drain side neighbor on WLn+1 experienced betweenthe time of write of WLn and the present time of read of WLn. For otherbit lines whose drain side neighbors are not being detected presently tobe in state C, the data of this read of WLn which used Vread1 on WLn+1will be disregarded.

In step 678, a read process is performed for WLn. During that readprocess, the drain side neighbor word line WLn+1 will receive Vread2.That is, VreadX=Vread2, where Vread2, as compared to Vread1, is closerin value to the Vreadp used during programming. This delivers a smallercompensation amount appropriate for cells whose drain side neighbors arenow in state B. One example of a compensation amount iscompB=Vread2−Vreadp=4.9−3=1.9V. Thus Vread2 differs from Vreadp bycompB. In step 680, the results of step 678 will be stored for bit lineswith memory cells having neighboring memory cells (e.g., WLn+1) in stateB. Data for other bit lines will be disregarded.

In step 682, a read process is performed for WLn. During that readprocess, the drain side neighbor word line WLn+1 will receive Vread3.That is, VreadX=Vread3, where Vread3, as compared to Vread2, is closerin value to the Vreadp used during programming. This delivers a yetsmaller compensation amount appropriate for cells whose drain sideneighbors are now in state A. One example of a compensation amount iscompA=Vread3−Vreadp=4.3−3=1.3 v. Thus Vread3 differs from Vreadp bycompA. In step 684, the results of step 682 will be stored for bit lineswith memory cells having neighboring memory cells (e.g., WLn+1) in stateA. Data for other bit lines will be disregarded.

In step 686, a read process is performed for WLn. During that readprocess, the drain side neighbor word line WLn+1 will receive Vread4.That is, VreadX=Vread4, where Vread4 is identical in value to Vreadpused during programming. This delivers no compensation amount which isappropriate for cells whose drain side neighbors are now in state E asthey were at the time of program/verify. This compensation amount iscompe=Vread4−Vreadp=3−3=0.0 v neighbor word line WLn+1 will receiveVread4. That is, VreadX=Vread4=Vread. In step 688, the results of step686 will be stored for bit lines with memory cells having neighboringmemory cells (e.g., WLn+1) in state E. Data for other bit lines will bedisregarded. During the process of FIG. 17, the neighboring bit linewill receive four voltages; however, each selected memory cell beingread will only make us of the one appropriate voltage.

In different implementations, different values of Vread1, Vread2, Vread3and Vread 4 can be determined based on device characterization,experimentation and/or simulation.

In the discussion above, the process of FIG. 17 is performed as part ofthe data recovery step 620 of FIG. 15. In another embodiment, theprocess of FIG. 17 can be used as the initial read process that isperformed in response to a request to read data. For example, afterreceiving a request to read data in step 598 of FIG. 15, the system willperform a read operation in step 600. In this embodiment, step 600 isimplemented by performing the process of FIG. 17. An embodiment thatuses the process of FIG. 17 to implement step 600 may not have theadditional data recovery step 620, so if an error is not correctable thesystem would report the error.

FIG. 18 is a flow chart indicating that the data recovery process (themethod of FIG. 17) can be performed for all the word lines of a blockexcept for the last word line to be programmed. For example, if thereare x+1 word lines, the recovery process can be used for word lines WL0through WLx−1. It would not be necessary to perform the recovery processfor word line WLx (e.g., the word line closest to the drain) becausethat word line has no neighbor that was programmed after it that wouldcause the floating gate to floating gate coupling effect. Although FIG.18 shows an embodiment with a recovery process performed for all theword lines sequentially, in one embodiment described above with respectto FIG. 15, the recovery process can be performed for the word lines atseparate times and only if there were ECC errors that were notcorrectable.

The above-described methods of FIGS. 16 and 17 were discussed withrespect to the full sequence programming storing two bits of one logicalpage of FIG. 11. These processes can be slightly modified when readingdata that was programmed according to the two-step process of FIG. 12storing one bit from each of two logical pages. For example, whenperforming the standard read operation (step 600 of FIG. 15), readingthe lower page would require applying Vra and Vrc to the control gatesof the memory cells and sensing at those read points to determinewhether the data is in state E/C (data 1) or states A/B (data 0) for thelower page. Thus, FIG. 16 would be modified by performing only steps640, 642, 644 and steps 652-660 for a lower page read. For performing aread of the upper page, read compare point Vrb would be used todetermine whether upper page data is for state E/A (data 1) or statesB/C (data 0). Therefore, for an upper page read, the process of FIG. 16would be amended to perform only steps 646, 648, 650, 658 and 660.Additionally, when recovering data (step 620), the process would performthe method of FIG. 19 for recovering data for a lower page and theprocess of FIG. 20 to recover data for an upper page.

In step 730 of FIG. 19, a read operation is performed for theneighboring word line WLn+1 according to the method of FIG. 16. In someembodiments, the read operation performed for WLn+1 results indetermining the actual data stored on WLn+1. In other embodiments, theread operation performed for WLn+1 results in a determination of chargelevels (or another condition) on WLn+1, which may or may not accuratelyreflect the data stored on WLn+1. The results of that read operation arestored in the appropriate latches in step 732. In step 734, a readoperation is performed for the word line of interest WLn, includingperforming the process of FIG. 10 with Vra being applied to WLn andVreadX=Vread4. In step 736, the data for the bit lines are sensed. Instep 738, the results are stored in the appropriate latches. In anotherembodiment of step 734, the read process would be performed withVreadX=Vread1. In one embodiment, the value of VreadX in step 734 shouldbe the same as used during the verification process.

In step 740, read reference voltage Vrc is applied to the word line WLnand a read operation is performed for the word line of interest WLn withVreadX=Vread1. In step 742, data is sensed as discussed above. In step744, the results of the sense step 742 will be stored for bit linesassociated with a neighboring cell storing data in state C.

In step 746, read reference voltage Vrc is applied to the word line WLnand a read operation is performed for the word line of interest WLn withVreadX=Vread2 for WLn+1. In step 948, the data will be sensed asdiscussed above. In step 950, the results of step 948 will be stored forbit lines associated with neighboring cells storing data in state B.Data for other bit lines will be discarded.

In step 752, read reference voltage Vrc is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread3 for WLn+1.In step 754, the data will be sensed as discussed above. In step 756,the results of step 754 will be stored for bit lines associated withneighboring cells storing data in state A. Data for other bit lines willbe discarded.

In step 758, read reference voltage Vrc is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread4 for WLn+1.In step 760, the data will be sensed as discussed above. In step 762,the results of step 760 will be stored for bit lines associated withneighboring cells storing data in state E. Data for other bit lines willbe discarded.

In step 764, processor 392 will determine the data values based on thedata stored from the sensing steps. In step 766, the determined datavalues from step 764 will be stored in latches for eventualcommunication to the user requesting the read of data. In anotherembodiment, steps 734-738 associated with state A could be performedbetween steps 762 and 764. Other orders for performing the steps of FIG.19, as well as the steps of other flow charts, can also be used.

Note that in the process described by FIG. 19, compensation is onlyapplied for Vrc in order to distinguish state B from state C. It isassumed that compensation is not needed when reading at Vra because theusually negative threshold of the erase state, though affected by WLn+1,is separated sufficiently far from state A as to not need correction.While this is a practical assumption for current generation memories, itmay not be true in future generation memories, and the compensationprocesses described with respect to Vrc may be used for Vra.

When determining the data values in step 764, if a memory cell conductsin response to Vra, the lower page data is “1.” If the memory cell doesnot conduct in response to Vra and does not conduct in response to Vrc,then the lower page data is also “1.” If the memory cell does notconduct in response to Vra, but does conduct in response to Vrc, thenthe lower page data is “0.”

The process of FIG. 20 is used to read or recover data for the upperpage. In step 800, a read operation is performed for the neighboringword line WLn+1 using the method of FIG. 16. In some embodiments, theread operation performed for WLn+1 results in determining the actualdata stored on WLn+1. In other embodiments, the read operation performedfor WLn+1 results in a determination of charge levels on WLn+1, whichmay or may not accurately reflect the data stored on WLn+1. In step 802,the results of step 800 are stored in the appropriate latches for eachof the bit lines.

In step 804, read reference voltage Vrb is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread1 for WLn+1.In step 806, the data will be sensed as discussed above. In step 808,the results of step 806 will be stored for bit lines associated withneighboring cells storing data in state C. Data for other bit lines willbe discarded.

In step 810, read reference voltage Vrb is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread2 for WLn+1.In step 812, the data will be sensed as discussed above. In step 814,the results of step 812 will be stored for bit lines associated withneighboring cells storing data in state B. Data for other bit lines willbe discarded.

In step 816, read reference voltage Vrb is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread3 for WLn+1.In step 818, the data will be sensed as discussed above. In step 820,the results of step 818 will be stored for bit lines associated withneighboring cells storing data in state A. Data for other bit lines willbe discarded.

In step 822, read reference voltage Vrb is applied to the word line WLnand a read operation is performed for WLn with VreadX=Vread4 for WLn+1.In step 824, the data will be sensed as discussed above. In step 826,the results of step 824 will be stored for bit lines associated withneighboring cells storing data in state E. Data for other bit lines willbe discarded.

In step 828, processor 392 determines the data values based on thestored sensed data. If a memory cell turned on in response to Vrb, thenthe upper page data is “1.” If a memory cell does not turn on inresponse to Vrb, then the upper page data is “0.” In step 830, the datavalues determined by processor 392 are stored in the data latches forcommunication to the user.

In another embodiment, rather than using the methods of FIGS. 19 and 20to recover data, the methods of FIGS. 19 and 20 can be used for theinitial data reads performed in response to a request to read data. Forexample, after receiving a request to read data in step 598 of FIG. 15,the system will perform a read operation in step 600. In thisembodiment, step 600 is implemented by performing the process of FIGS.19 and/or 20. An embodiment that uses the process of FIGS. 19 and/or 20to implement step 600 may not have the additional data recovery step620, so if an error is not correctable the system would report theerror.

FIGS. 19 and 20 are for reading data that are programmed using the upperpage and lower page process of FIG. 12. These two methods of FIGS. 19and 20 can be used to read data programmed by all bit line programmingor odd/even bit line programming. When used with all bit lineprogramming, all bit lines are typically read simultaneously. When usedwith odd/even bit line programming, even bit lines are typically readsimultaneously at a first time and odd bit lines are typically readsimultaneously possibly at a different time.

FIGS. 21-26 describe processes used to read data that is programmedaccording to the method associated with FIGS. 13A-C. The process of FIG.21 can be implemented as an overall process for reading data that isperformed in response to a read request for a particular one or morepages (or other grouping) of data prior to, separate from and/or inconjunction with using ECCs. In other embodiments, the process of FIG.21 can be performed as part of data recovery step 620 of FIG. 15. Whenreading data as programmed according to the process of FIGS. 13A-C, anyperturbation from floating gate to floating gate coupling due toprogramming the lower page of neighboring cells should be corrected whenprogramming the upper page of the memory cell under question. Therefore,when attempting to compensate for floating gate to floating gatecoupling effect from neighboring cells, one embodiment of the processneed only consider the coupling effect due to the programming of theupper page of neighboring cells. Thus, in step 1060 of FIG. 21, theprocess reads upper page data for the neighboring word line. If theupper page of the neighboring word line was not programmed (step 1062),then the page under consideration can be read without compensating forthe floating gate to floating gate coupling effect (step 1064). If theupper page of the neighboring word line was programmed (step 1062), thenthe page under consideration should be read using some compensation forthe floating gate to floating gate coupling effect in step 1066. In someembodiments, the read operation performed for neighboring word lineresults in a determination of charge levels on the neighboring wordline, which may or may not accurately reflect the data stored thereon.Also, note that the selected word line to be read, i.e. WLn, may itselfhave only lower page data. This can happen when the entire block has notyet been programmed. In such a situation it is always guaranteed thatthe cells on WLn+1 are still erased, and therefore, no coupling effecthas yet plagued WLn cells. This means that no compensation is required.So the lower page read of a word line whose upper page has yet to beprogrammed can proceed as usual without the need for any compensationtechnique.

In one embodiment, a memory array implementing the programming processof FIGS. 13A-C will reserve a set of memory cells to store one or moreflags. For example, one column of memory cells can be used to storeflags indicating whether the lower page of the respective rows of memorycells has been programmed and another column of memory cells can be usedto store flags indicating whether the upper page for the respective rowsof memory cells has been programmed. In some embodiments, redundantcells can be used to store copies of the flag. By checking theappropriate flag, it can be determined whether the upper page for theneighboring word line has been programmed. More details about such aflag and the process for programming can be found in U.S. Pat. No.6,657,891, Shibata et al., “Semiconductor Memory Device For StoringMulti-Valued Data,” incorporated herein by reference in its entirety.

FIG. 22 describes one embodiment of a process for reading the upper pagedata for a neighboring word line such as the drain side neighbor (step1060 of FIG. 21). In step 1100, read reference voltage Vrc is applied tothe word line associated with the page being read. At step 1102, the bitlines are sensed as described above. In step 1104, the results of step1102 are stored in the appropriate latches. In step 1106, the systemchecks the flag indicating upper page programming associated with thepage being read. In one embodiment, the memory cell storing the flagwill store data in state E if the flag is not set and in state C if theflag is set. Therefore, when that particular memory cell is sensed atstep 1102, if the memory cell conducts (turns on), then the memory cellis not storing data in state C and the flag is not set. If the memorycell does not conduct, then it is assumed in step 1106 that the memorycell is indicating that the upper page has been programmed.

In another embodiment, the flag can be stored in a byte. Rather thanstoring all bits in state C, the byte will include a unique 8-bit coderepresenting the flag and known to the state machine 312, such that the8-bit code has at least one bit in state E, at least one bit in state A,at least one bit in state B and at least one bit in state C. If theupper page has not been programmed, the byte of memory cells will all bein state E. If the upper page has been programmed, then the byte ofmemory cells will store the code. In one embodiment, step 1106 isperformed by checking whether any of the memory cells of the bytestoring the code do not turn on in response to Vrc. In anotherembodiment, step 1106 includes addressing and reading the byte of memorycells storing the flag and sending the data to the state machine, whichwill verify whether the code stored in the memory cells matches the codeexpected by the state machine. If so, the state machine concludes thatthe upper page has been programmed.

If the flag has not been set (step 1108), then the process of FIG. 22terminates with the conclusion that the upper page has not beenprogrammed. If the flag has been set (step 1108), then it is assumedthat the upper page has been programmed and at step 1120 voltage Vrb isapplied to the word line associated with the page being read. In step1122, the bit lines are sensed as discussed above. In step 1124, theresults of step 1122 are stored in the appropriate latches. In step1126, voltage Vra is applied to the word line associated with the pagebeing read. In step 1128, the bit lines are sensed. In step 1130, theresults of step 1128 are stored in the appropriate latches. In step1132, processor 392 determines the data value stored by each of thememory cells being read based on the results of the three sensing steps1102, 1122 and 1128. At step 1134, the data values determined in step1132 are stored in the appropriate data latches for eventualcommunication to the user. In step 1132, processor 392 determines thevalues of the upper page and lower page data using well known simplelogic techniques dependent on the specific state coding chosen. Forexample, for the coding described in FIG. 13, the lower page data isVrb* (the complement of the value stored when reading at Vrb), and theupper page data is Vra* OR (Vrb AND Vrc*).

In one embodiment, the process of FIG. 22 includes the application ofVread to the drain side neighboring word line. Therefore, VreadX=Vreadfor the process of FIG. 22. In another embodiment of the process of FIG.22, VreadX=Vread4.

FIG. 23 is a flow chart describing one embodiment of a process forreading data of the word line under consideration when the system doesnot need to compensate for floating gate to floating gate coupling froma neighboring word line (see step 1064 of FIG. 21). In step 1150, it isdetermined whether the read is for the upper page or lower pageassociated with the word line under consideration. If the read is forthe lower page, then in step 1152 voltage Vrb is applied to the wordline associated with the page being read. In step 1154, the bit linesare sensed. In step 1156, the results of sensing step 1154 are stored inthe appropriate latches. In step 1158, the flag is checked to determineif the page contains upper page data. If there is no flag, then any datapresent will be in the intermediate state and Vrb was the incorrectcomparison voltage to use and the process continues at step 1160. Instep 1160, Vra is applied to the word line, the bit lines are re-sensedat step 1162, and in step 1164 the result is stored. In step 1166 (aftereither step 1164, or step 1158 if the flag is set, processor 392determines a data value to be stored. In one embodiment, when readingthe lower page, if the memory cell turns on in response to Vrb (or Vra)being applied to the word line, then the lower page data is “1”;otherwise, the lower page data is “0.”

If it is determined that the page address corresponds to the upper page(step 1150), an upper page read process is performed at step 1170. Inone embodiment, the upper page read process of step 1170 includes thesame method described in FIG. 22, which includes reading the flag andall three states since an unwritten upper page may be addressed forreading, or another reason.

In one embodiment, the process of FIG. 23 includes the application ofVread to the drain side neighboring word line. Therefore, VreadX=Vreadfor the process of FIG. 23. In another embodiment of the process of FIG.22, VreadX=Vread4.

FIG. 24 depicts a flow chart describing one embodiment of a process forreading data while compensating for floating gate to floating gatecoupling effect (see step 1066 of FIG. 21). In step 1200 of FIG. 24, thesystem determines whether to use compensation for the floating gate tofloating gate coupling. This is performed separately for each bit line.The appropriate processor 392 will determine which bit lines need to usethe compensation based on the data from the neighboring word lines. If aneighboring word line is in state E or B (or has charge apparentlyindicating state E or B), then the particular word line being read neednot compensate for the floating gate to floating gate coupling effect.The assumption is that if it is in state E it hasn't contributed to anycoupling because the threshold hasn't moved since the current word linewas written. If it is in state B, it got there from B′, and the movementfrom B′ to B is small and can be neglected. In another embodiment, thissmall movement can be compensated for by the application of aproportionately small ΔVREAD.

In one embodiment, the process of step 1200 can be performedconcurrently with step 1060. For example, FIG. 25 provides a chartexplaining steps to perform a determination whether to use an offset fora particular bit line. The first step is to perform a read process usingVra on the word line. The second step is to perform a read using Vrb.When reading at Vra, a latch stores a 1 if the memory cell is in state Eand a 0 if the memory cell is in states A, B, C or. When reading at Vrb,the latch will store a 1 for states E and A, and store a 0 for states Band C. The third step of FIG. 25 includes performing an XOR operation onthe inverted results from the second step with the results from step 1.In the fourth step, a read is performed using Vrc at the word line. Alatch stores a 1 for states E, A and B, and stores a 0 for state C. Inthe fifth step, the results of step 4 and step 3 are operated by alogical AND operation. Note that steps 1, 2 and 4 may be performed aspart of FIG. 22. Steps 3 and 5 of FIG. 25 can be performed by dedicatedhardware or by processor 392. The results of step 5 are stored in alatch with 1 being stored if no compensation is needed and 0 beingstored if compensation is needed. Thus, a compensation will be requiredfor those cells that are read on WLn that have neighboring memory cellson WLn+1 that are in the A or C state. This approach requires only onelatch to determine whether to correct WLn or not, in contrast to someprevious methods that store the full data from WLn+1, requiring two ormore latches.

Looking back at step 1202 of FIG. 24, it is determined whether the pagebeing read is the upper page or lower page. If the page being read isthe lower page, then Vrb is applied to the word line WLn associated withthe page being read and Vread4 is applied to the drain side neighborword line WLn+1 during a read process in step 1204. Note that for thestate coding described in FIG. 13, reading at Vrb is sufficient todetermine the lower page data. In step 1208, the results of step 1206are stored in the appropriate latches associated with the bit lines. Instep 1210, Vrb will be applied to the word line WLn for the page beingread and Vread3 is applied to the drain side neighbor word line WLn+1during a read process (e.g., see FIG. 10). In step 1212, the bit linesare sensed. In step 1214, the results of the sensing of step 1212 areused to overwrite the results stored in step 1208 for the bit lines forwhich it was determined at step 1200 to use compensation. If theparticular bit line is determined not to have to use compensation, thenthe data from step 1212 is not stored. In step 1216, processor 392 willdetermine whether the data is 1 or 0 for the lower page. If the memorycell turned on in response to Vrb, then the lower page data is 1;otherwise, the lower page data is 0. At step 1218, the lower page datais stored in the appropriate latches for communication to the user.

If it is determined at step 1202 that the page being read is the upperpage, then the upper page correction process is performed at step 1220.FIG. 26 provides a flow chart describing the upper page correctionprocess. In step 1250 of FIG. 26, read reference voltage Vrc is appliedto the word line associated with the page being read and Vread4 isapplied to the drain side neighbor word line WLn+1 as part of a readprocess. In step 1252, the bit lines are sensed. In step 1254, theresults of the sensing step are stored in the appropriate latches. Instep 1256, Vrc is applied to the word line associated with the pagebeing read and Vread3 is applied to the drain side neighbor word lineWLn+1 as part of a read process. In step 1258, the bit lines are sensed.In step 1260, the results of the sensing step 1258 are used to overwritethe results stored in step 1254 for any bit line for which thecompensation is required (see step 1200).

At step 1270, Vrb is applied to the word line and Vread4 is applied tothe drain side neighbor word line WLn+1 during a read process. In step1272, the bit lines are sensed. In step 1274, the results of sensingstep 1272 are stored. In step 1276, Vrb is applied to the word lineassociated with the page being read and Vread3 is applied to the drainside neighbor word line WLn+1 during a read process. In step 1278, thebit lines are sensed. In step 1280, the results of step 1278 are used tooverwrite the results stored at step 1274 for those bit lines for whichthe compensation is required (see step 1200).

In step 1282, Vra is applied to the word line associated with the pagebeing read and Vread4 is applied to the drain side neighbor word lineWLn+1 as part of a read process. In step 1284, the bit lines are sensed.In step 1286, the results of the sensing step 1284 are stored in theappropriate latches. In step 1288, Vra is applied to the word lineassociated with the page being read and Vread3 is applied to the drainside neighbor word line WLn+1 as part of a read process. In step 1290,the bit lines are sensed. In step 1292, the results of step 1290 areused to overwrite the results stored in step 1286 for those bit linesfor which the compensation is required (see step 1200). In step 1294,the processor 392 determines the data values in the same manner aspreviously described another method known in the art. In step 1296, thedata values determined by the processor 392 are stored in theappropriate data latches for communication to the user. In otherembodiments the order of reading (Vrc, Vrb, Vra) may be changed.

In the above discussion with respect to FIG. 21, an example is discussedinvolving the reading of a page of data. It is likely, but not required,that a request to read data will require the reading of multiple pagesof data. In one embodiment, to speed up the process of reading multiplepages of data, the read process will be pipelined such that the statemachine will execute a next page sensing while the user is transferringout the previous page of data. In such an implementation, the flag fetchprocess may interrupt the pipelined read process. To avoid such aninterruption, one embodiment contemplates reading the flag for a givenpage when that page is read and using the wired-OR detection process tocheck the flag (rather than reading the flag and sending it to the statemachine). For example, during step 1060 of FIG. 21 (reading theneighboring word line), the process first reads data using Vrc as thereference voltage. At that point, if the wired-OR line indicates thateach state stores data 1, then the upper page has not been programmed;therefore, no compensation is needed and the system will read withoutcompensating for the floating gate to floating gate coupling (step1064). If the flag is a one-byte code that includes data in each datastate, at least the flag memory cells would have data in state C if theflag is set. If the wired-OR line indicates that no memory cells havedata in state C, then the state machine concludes that the flag has notbeen set; therefore, the upper page for the neighboring word line hasnot been programmed and compensation for floating gate coupling is notneeded. More information about performing pipelined reads can be foundin U.S. patent application Ser. No. 11/099,133, titled “Compensating forCoupling During Read Operations of Non-Volatile Memory,” Inventor JianChen, filed on Apr. 5, 2005, incorporated herein by reference in itsentirety.

The above-described techniques help to reverse the effects of thefloating gate to floating gate coupling. FIG. 27 graphically explainsthe concept of floating gate to floating gate coupling. FIG. 27 depictsneighboring floating gates 1302 and 1304, which are on the same NANDstring. Floating gates 1302 and 1304 are situated above NANDchannel/substrate 1306, which has source/drain regions 1308, 1310 and1312. Above floating gate 1302 is control gate 1314 that is connected toand part of word line WLn. Above floating gate 1304 is control gate 1316that is connected to and part of word line WLn+1. Although floating gate1302 will likely be subject to coupling from multiple other floatinggates, for simplicity FIG. 27 only shows the effects from oneneighboring memory cell. Specifically, FIG. 27 shows three components ofcoupling provided to floating gate 1302 from its neighbor: r1, r2 andCr. The component r1 is the coupling ratio between the neighboringfloating gates (1302 and 1304), and is calculated as the capacitance ofthe neighboring floating gates divided by the sum of all capacitivecouplings of floating gate 1302 to all the other electrodes surroundingit. The component r2 is the coupling ratio between the floating gate1302 and the drain side neighbor control gate 1316, and is calculated asthe capacitance of floating gate 1302 and control gate 1316 divided bythe sum of all capacitive couplings of floating gate 1302 to all theother electrodes surrounding it. The component Cr is the control gatecoupling ratio and is calculated as the capacitance between floatinggate 1304 and its corresponding control gate 1316 divided by the sum ofall capacitive couplings of floating gate 1302 to all the otherelectrodes surrounding it.

In one embodiment, the amount of required compensation, ΔVread, can becalculated as follows:

${\Delta \; {Vread}} = {\left( {{\Delta \; {VTn}} + 1} \right)\frac{1}{1 + \frac{r\; 2}{\left( {r\; 1} \right)({Cr})}}}$

Where ΔVTn+1 is the change in threshold voltage of the drain sideneighbor memory cell between the time of program/verify of WLn and thepresent time. ΔVTn+1, and r1 are the root causes of the word line toword line parasitic coupling effect that is mitigated by the presentmethod. ΔVread is the compensation that is brought to bear in order tocombat this effect.

Compensation for coupling described herein can be achieved by utilizingthe same parasitic capacitance between neighboring floating gates aswell as capacitance between the floating gate and the neighboringcontrol gate. Since the control gate/floating gate stack is typicallyetched in one step, the compensation tracks the variations in spacingfrom memory cell to memory cell. Thus, when two neighbors are fartherapart, the coupling is smaller and so will the required compensation forthis effect be naturally smaller. When two neighbors are closer, thecoupling is larger and so is compensation larger. This constitutesproportional compensation.

The above-described compensation also reduces the effects of variationsin etch back depth. In some devices, the control gate partially wrapsaround the floating gate. The amount of overlap is called “etch back.”Variations in etch back depth can effect the amount of coupling. Withthe above-described compensation scheme, the effect of the compensationwill similarly vary with etch back depth.

As a result of the ability to reduce the effects of the floating gate tofloating gate coupling, the margins between threshold voltagedistributions can be made smaller or the memory system can programfaster.

Another important advantage of the present method is that the resolutionof the digital to analog converters that drive the voltages on WLn,and/or WLn+1 does not have to be as fine for the present invention incomparison to some prior art which achieves the compensation throughchanging voltages applied to selected word line WLn. The change requiredfor compensation when the compensation is applied to the selected wordline has to be much more refined in comparison to the present inventionwhere the change acts indirectly through parasitic couplings andtherefore a much coarser resolution of Vread will translate into a muchfiner equivalent resolution of WLn margining voltage.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for reading data from non-volatile storage, comprising:reading from a neighboring word line of a selected word line, saidselected word line is connected to a set of selected non-volatilestorage elements, said reading from a neighboring word line includesdetermining conditions of non-volatile storage elements that areneighbors to said set of selected non-volatile storage elements;performing at least two read processes for said selected word lineincluding applying a different voltage to said neighboring word line foreach of said at least two read process; and individually choosing datafrom one of said read processes for each of said set of selectednon-volatile storage elements based on conditions of said non-volatilestorage elements that are neighbors to said set of selected non-volatilestorage elements.
 2. A method according to claim 1, wherein: saidperforming at least two read processes includes performing one readprocess for each potential state of said non-volatile storage elementsthat are neighbors to said set of selected non-volatile storageelements.
 3. A method according to claim 1, wherein: said performing atleast two read processes includes applying a pass voltage tonon-selected word lines for said at least two read processes, said passvoltage is different than at least one voltage concurrently applied tosaid neighboring word line during said read processes.
 4. A methodaccording to claim 1, wherein: said non-volatile storage element beingread is a multi-state NAND flash memory device.
 5. A method for readingdata from a set of selected non-volatile storage elements, eachnon-volatile storage element of said set has a respective neighboringnon-volatile storage element, said method comprising: sensing conditioninformation for said respective neighboring non-volatile storageelements; performing a first read process for said selected non-volatilestorage elements, said first read process includes applying a readvoltage to said selected non-volatile storage elements and a first passvoltage to said respective neighboring non-volatile storage elements;performing a one or more additional read processes for said selectednon-volatile storage elements, said one or more additional readprocesses includes applying said read voltage to said selectednon-volatile storage elements and additional pass voltages to saidrespective neighboring non-volatile storage elements; and individuallyfor each of said selected non-volatile storage elements, choosing datafrom said first read process or said additional processes based on saidcondition information for said respective neighboring non-volatilestorage elements.
 6. A method according to claim 5, wherein: said set ofselected non-volatile storage elements and said respective neighboringnon-volatile storage elements are part of NAND strings; said NANDstrings also include other unselected non-volatile storage elements; andsaid first read process and said one or more additional read processesinclude applying a particular pass voltage to other unselectednon-volatile storage elements, said particular pass voltage is differentthan at least one of said additional pass voltages.
 7. A methodaccording to claim 5, wherein said one or more additional read processescomprise: performing a second read process for said selectednon-volatile storage elements, said second read process includesapplying said read voltage to said selected non-volatile storageelements and a second pass voltage to said respective neighboringnon-volatile storage elements, said step of choosing data includeschoosing data from said first read process or said second read processbased on said condition information for said respective neighboringnon-volatile storage elements.
 8. A method according to claim 5, whereinsaid one or more additional read processes comprise: performing a secondread process for said selected non-volatile storage elements, saidsecond read process includes applying said read voltage to said selectednon-volatile storage elements and a second pass voltage to saidrespective neighboring non-volatile storage elements; performing a thirdread process for said selected non-volatile storage elements, said thirdread process includes applying said read voltage to said selectednon-volatile storage elements and a third pass voltage to saidrespective neighboring non-volatile storage elements; and performing afourth read process for said selected non-volatile storage elements,said fourth read process includes applying said read voltage to saidselected non-volatile storage elements and a fourth pass voltage to saidrespective neighboring non-volatile storage elements; said step ofchoosing data includes choosing data from said first read process, saidsecond read process, said third read process or said fourth read processbased on said condition information for said respective neighboringnon-volatile storage elements.
 9. A method for reading data non-volatilestorage, comprising: providing a compare voltage to a selectednon-volatile storage element; sensing a condition of a non-volatilestorage element neighboring said selected non-volatile storage element;providing compensation to said non-volatile storage element neighboringsaid selected non-volatile storage element based on said condition; andsensing data for said a selected non-volatile storage element inresponse to said compare voltage and said compensation.